The reduction of the melting point in nanometer size metal and molecular crystals is well established experimentally (Ph. Buffat and J.-P. Borel Phys. Rev. A (1976) 13:2287; M. Y. Hahn and R. L. Whetten, Phys. Rev. Lett. (1988) 61:1190, and is the subject of numerous theoretical studies and simulations (P. Sheng, R. W. Cohen, and J. R. Schrieffer, J. Phys. (1981) C 14:L565; R. S. Berry, J. Jellinek, and G. Natanson Phys. Rev. A (1984) 30:919; J. D. Honeycutt and H. C. Andersen, J. Phys. Chem. (1987) 91:4950). It is of interest to extend these studies of melting to other classes of materials. In particular, systems in which the bonding is more covalent and the structure more open may behave differently. Also, studies of covalent systems would extend current models of melting which rely on the concept of surface tension, a term difficult to define for a covalent cluster. Finally, the study of melting in any cluster material lends insight into the stability of these metastable systems, an issue vital to the development of new synthetic methods
Despite the many reasons to study melting in clusters, the field has been until recently limited to metals and molecular crystals. New developments in chemical methods of preparation have enabled the synthesis of narrow size distributions of highly crystalline, nanometer size, crystallites of inorganic semiconductors like CdS and GaAs (M. L. Steigerwld et al., J. Am. Chem. Soc. (1988) 110:3046; M. A. Olshavsky, A. N. Goldstein, and A. P. Alivisatos, "Organometallic Synthesis of GaAs Crystallites Exhibiting Quantum Confinement," J. Am. Chem. Soc., in press). These materials are polar semiconductors and have open structures. They are large enough to possess the bulk zincblende crystal structure, but a large fraction of the atoms are on the surface. Hence, as in the metal clusters, it is expected that the surface will play a dominant role in their high temperature behavior.
We have recognized that a useful application of this effect could be in the formation of thin-film of conductors and semiconductors in chip fabrication and the like in that the lower temperatures make it possible to form thin film with less thermal hazard to thermally sensitive underlayers, substrates and other structures.